The fish paste production process includes a water leaching step for leaching water from fish meat. This step makes it possible to remove substances contained in fish meat that inhibit gel formation, thereby improving the elasticity of the fish paste. However, water leaching causes the overflow of large amounts of water-soluble proteins in fish meat into leaching water, which has been problematic in terms of the effective use of natural resources and water treatment of leaching waste water.
A variety of methods for recovering proteins that have been discharged into fish paste leaching waste water by insolubilizing the proteins and recovering the insolubilized proteins via solid-liquid separation in a step of fish meat water leaching have been developed.
For instance, two-stage pH shift methods, which comprise acidizing fish paste leaching waste water, neutralizing the water to insolubilize water-soluble proteins, and recovering the insolubilized proteins, have been reported (see Non-Patent Documents 1-4). Proteins that have been insolubilized via two-stage pH shift methods to flocculate (form flocs) can be easily separated using a centrifuge at the laboratory level. However, since such proteins have low flocculation capacity, it has been difficult to carry out continuous solid-liquid separation at the factory level.
In view of the above, solid-liquid separation methods involving pressure flotation, which comprise allowing proteins to flocculate with the use of a inorganic flocculation agent such as polyaluminum chloride (PAC) or a synthetic polymer flocculation agent, have been developed (see Patent Documents 1-5). However, these methods are limited in terms of the usage of recovered proteins because of the use of flocculation agents. In addition, separation via pressure flotation results in large moisture content in the recovered product, which in turn requires a dewatering step. Accordingly, yield reduction and further quality deterioration of waste liquid are problematic.
Meanwhile, in addition to the above methods for recovering proteins in fish paste waste liquid, a method for removing, as unwanted matter, proteins in waste water generated during starch production has been developed, the method comprising subjecting waste water to isoelectric point treatment at pH 3.5-5 or heating at 60° C. to 90° C. to insolubilize unnecessary proteins (see Patent Document 6). Since the object of this method was not the effective use of insolubilized proteins, changes in physical properties of insolubilized proteins were not taken into account.